Streamflow of 2012 - Water Year Summary

Introduction

The maps and graphs in this summary describe
streamflow conditions for water-year 2012 (October 1, 2011 to September 30,
2012) in the context of the 83-year period 1930-2012, unless otherwise noted.
The illustrations are based on observed data from the U.S. Geological
Survey's (USGS) National Streamflow Information Program (http://water.usgs.gov/nsip). The period 1930-2012
was used because prior to 1930, the number of streamgages was too small to
provide representative data for computing statistics for most regions of the
country.

In the summary, reference is made to the term "runoff," which is
the depth to which a river basin, State, or other geographic area would be
covered with water if all the streamflow within the area during a specified time period was uniformly distributed upon it. Runoff quantifies the magnitude of water
flowing through the Nation's rivers and streams in measurement units that can
be compared from one area to another.

Each of the maps and graphs below can be expanded to a larger view by
clicking on the image. In all the graphics, a rank of 1 indicates the highest
flow of all years analyzed.

National Overview

Average runoff in the Nation’s rivers and streams during 2012
(7.90 inches) was lower than the long-term annual mean for the
United States (9.30 inches). Nationwide, 2012 streamflow ranked 69th out of the 83 years in the period 1930-2012. Note that in previous water-year summaries (prior to 2011) the median runoff, not the average runoff, was compared among time periods.

Streamflow was at record low levels (ranking 83rd in 83 years) in Georgia. Below normal and much-below normal streamflow was prevalent cross the Southeast, Midwest, and Southwest. Above normal streamflow occurred only in Ohio. Most states had streamflow in the normal range.

* For Puerto Rico, 69 years of available data were used.

Regional Patterns

The United States (including Puerto Rico) is divided into 21 large
drainages, or water resources regions. These hydrologic areas are
based on surface topography and contain either the drainage area of
a major river, such as the Columbia, the combined drainage areas of
a series of rivers, such as the Texas-Gulf region which includes a
number of rivers draining into the Gulf of Mexico, or the area of
an island or island group. Water resources regions provide a coherent,
watershed-based framework for depicting streamflow variations.

Streamflows at much-below normal levels were reported in the South Atlantic-Gulf, Rio Grande, and Lower Colorado regions. Below normal conditions were measured in the Upper Mississippi and Upper Colorado regions.

Seasonal Characteristics

Autumn season (October-December) streamflow was at record high levels (ranking 1st in 83 years) in Ohio. Above and much-above normal flows were reported in the Northeast, Great Lakes and northern states, as well as a few western states. West-coast states, Texas, Louisiana, Georgia, and South Carolina had below and much-below normal streamflows. Nationwide, autumn season streamflow ranked 21st out of 83 years.

* For Puerto Rico, 69 years of available data were used.

Winter season (January-March) streamflow was at record low levels in Georgia and Florida. Above or much-above normal streamflow was reported in east-coast states, California, Arizona, and Iowa. Above normal conditions occurred in Wyoming, Utah, Louisiana, as well as Puerto Rico. Nationwide, winter season streamflow ranked 52nd out of 83 years.

*For Puerto Rico, 69 years of available data were used.

Spring season (April-June) streamflow was below or much-below normal in most states (37 states). Above-normal streamflow was seen only in Washington, Oregon, and Puerto Rico. On a nationwide basis, spring season streamflow was much-below normal, ranking 81st in 83 years.

* For Puerto Rico, 69 years of available data were used.

Summer season (July-September) streamflow in midwestern and southeastern states -- as well as Delaware, Vermont and New Hampshire -- was below or much-below normal. Streamflow in Kansas and Nebraska was at record low levels. Streamflow was above normal in Washington, Louisiana, and Florida. On a nationwide basis, summer season streamflow was much-below normal, ranking 76th in 82 years.

* For Puerto Rico, 69 years of available data were used.

High and Low Flows

In any given month, on average, it is expected that five
percent of the streamgages will experience very high (>95th percentile)
and very low (<5th percentile) average streamflow. During
water year 2012, only three fall months (October, November and December) had a greater than expected percentage of
streamgages reporting very high streamflow (8, 7, and 8
percent, respectively). In contrast, there were 8 months (from February to September) with a
greater than expected percentage of streamgages with very low flows (6, 6, 18, 8, 13, 13, 10, and 9 percent, respectively).

The bankfull streamflow is defined as the highest daily mean
streamflow value expected to occur, on average, once in every 2.3 years. In
2012, 23 percent of streamgages had a daily mean streamflow value above the
bankfull level. This value is much smaller than the expected number (43 percent) to
occur in any given year. Since 1950, the largest number of streamgages
reporting higher than bankfull streamflow in any one year was 69 percent,
which occurred in 1996.

The 10th percentile 7-day low flow is defined
as the lowest 7-day average streamflow expected to occur, on average, once in
every 10 years. In water-year 2012, 12 percent of the streamgages reported a
7-day low flow less than the 10th percentile 7-day low flow value. The
expected number to occur in any given year is 10 percent. Since 1950, the
largest percentage of streamgages reporting a 7-day low flow less than the
10th percentile 7-day low flow was 25 percent, which occurred in 1954.

Additional Information

The USGS operates a network of nearly 10,000 streamgages nationwide,
most in real-time. Current information derived from these stations
is available on the web at http://waterwatch.usgs.gov.
Tables of data that summarize historical streamflow conditions by
State, beginning in the year 1900, can be accessed at http://waterwatch.usgs.gov/?m=statesum.
These tables are updated every few months to reflect the most current
streamflow data.

The streamflow information used to prepare this summary is also used
for water management, monitoring floods and droughts, bridge design,
and for many recreational activities. To obtain real-time and archived
streamflow data and information, visit http://water.usgs.gov/nwis.
Although the national streamgage network is operated primarily by
the USGS, it is funded by a partnership of 850 agencies at the Federal,
State, Tribal, and local levels. For more information about the streamgage
network, see http://water.usgs.gov/nsip/.